The present invention relates in general to pressurized fluidized bed boilers, and in particular, to a new and useful depressurization system for quickly and safely depressurizing the pressure vessel of such boilers.
Various types of fluidized bed boilers exist which are distinguished by the pressure and velocity of fluidizing gas used in the boiler. Bubbling fluidized bed boilers utilize relatively low fluidizing gas velocities. Higher gas velocities are used in turbulent fluidized bed boilers. Even greater gas velocities are used in fast fluidized bed boilers.
A Babcock and Wilcox Technical Paper entitled "The Fast Fluidized Bed--A True Multi-Fuel Boiler" by Lars Stromberg et al, presented to the Eight International Conference on Fluidized-Bed Combustion in Houston, Tex., on Mar. 18-21, 1985, outlines the different types of fluidized bed boilers.
Fluidized bed boilers are also distinguished by the pressure at which they operate. Atmospheric fluidized bed boilers operate at or near atmospheric pressure. These type of boilers do not require an enclosure in the form of a pressure vessel. Pressurized fluidized bed boilers, on the other hand, operate at elevated pressures. U.S. Pat. No. 4,584,949 to Brannstrom discloses a pressurized fluidized bed reactor having a combustion chamber and a storage container housed within a pressure vessel. The upper end of the container is provided with a pressure relief line and valve and a conduit which is connected to receive bed material from the combustion chamber. To remove bed material from the combustion chamber, the pressure relief valve is opened. The internal pressure in the combustion chamber then forces bed material into the container. A throttle valve is connected to the pressure relief line so that any leakage through the relief valve is satisfied by the pressurized gas within the container, rather than by the gas of the combustion chamber. However, this reference does not disclose a depressurization system which can be used as a safety feature for quickly and efficiently depressurizing the pressure vessel in case of an emergency.
Fast fluidized bed boilers which are also operated at elevated temperatures are disclosed by U.S. Pat. No. 4,538,549 to Stromberg and U.S. Pat. No. 4,548,138 to Korenberg.
U.S. Pat. No. 4,546,709 to Astrom discloses a pressurized, fluidized bed reactor having a cyclone separator arrangement for removing particles from the hot exhaust gases of a combustor so that the exhaust gases can be lead to a turbine to drive the turbine. This patent, which is incorporated herein by reference, teaches the enclosure of the furnace within a vessel operated at super-atmospheric pressure.
Hot gas blowoff systems are known to utilize valves that can be opened to vent high temperature and dust-laden gases from the combustion chamber within the pressurized vessel. Cold depressurization systems are also known which vent hot air from the pressure vessel to the atmosphere.